A runner competes in the Comrades Marathon, South Africa Reuters

Overexerting yourself with exercise can lead to blood poisoning, a team of researchers have claimed.

Scientists from Monash University in Melbourne, Australia, say that pushing your body to the limit can cause intestinal bacteria to leak into the bloodstream.

The team, led by Dr Ricardo Costa from the Department of Nutrition and Dietetics, monitored people who participated in lengthy endurance events – such as 24-hour ultra-marathons and multi-stage ironman contests – over consecutive days.

They found that, when comparing before and after blood samples, prolonged exercise "causes the gut wall to change, allowing the naturally present bacteria, known as endotoxins, in the gut to leak into the bloodstream". The result of this is that the body triggers its systemic inflammatory response from the body's immune cells – which is similar to a serious infection.

However, athletes – whether professional or hobbyists – who are healthy develop immune characteristics that help combat any exercise-related infection when they follow a consistent training program in anticipation of endurance events.

But those who have had little training "put their bodies under enormous strain above the body's protective capacity" in such events says Dr Costa, warning that as little as four hours tough exercise for consecutive days could be dangerous.

In a worst case scenario, this can lead to sepsis-induced systemic inflammatory response syndrome, which can be fatal if not treated immediately.

"Nearly all of the participants in our study had blood markers identical to patients admitted to hospital with sepsis," said Dr Costa. "That's because the bacterial endotoxins that leak into the blood as a result of extreme exercise trigger the body's immune cells into action.

"Exercising in this way is no longer unusual – waiting lists for marathons, Ironman Triathlon events and ultra-marathons are the norm and they're growing in popularity.

"It's crucial that anyone who signs up to an event gets a health check first and builds a slow and steady training program, rather than jumping straight into a marathon, for example, with only a month's training.

"The body has the ability to adapt and put a brake on negative immune responses triggered by extreme endurance events. But if you haven't done the training and you're unfit – these are the people who can get into trouble."

One of the University of Monsah's research pieces, a 24-hour ultra-marathon study, was published in the International Journal of Sports Medicine, while a multi-stage ultra-marathon study was published in Exercise Immunology Reviews.